The present invention relates to a solid-state image sensor for obtaining a two-dimensional image using a photoelectric conversion effect.
Prior art will be described with reference to the drawings.
FIG. 1 is an example of the circuit diagram of a solid-state image element called xe2x80x9can amplifying MOS sensorxe2x80x9d.
In FIG. 1, unit cells 3xc3x973 consisting of amplifying transistors 2-1-1, 2-1-2, . . . , 2-3-3 for reading signals of photodiodes 1-1-1, 1-1-2, . . . , 1-3-3, vertical select transistors 3-1-1, 3-1-2, . . . , 3-3-3 for selecting lines for reading signals and reset transistors 4-1-1, 4-1-2, . . . , 4-3-3 for resetting signal charges are arranged two-dimensionally.
It is noted that more unit cells are arranged in the actual device. Horizontal address lines 6-1, . . . , 6-3 connected to a vertical shift register 5 in the horizontal direction are connected to gates of vertical select transistors 3-1-1, 3-1-2, . . . , 3-3-3, respectively, to determine a line for reading a signal. Reset lines 7-1, . . . , 7-3 are connected to gates of reset transistors 4-1-1, 4-1-2, . . . , 4-3-3, respectively. Sources of the amplifying transistors 2-1-1, 2-1-2, . . . , 2-3-3 are connected to vertical signal lines 8-1, . . . , 8-3, respectively and load transistors 9-1, . . . , 9-3 are provided on one ends of the sources thereof, respectively. Other ends of the vertical signal lines 8-1, . . . , 8-3 are connected to a horizontal signal line 11 through horizontal select transistors 19-1, . . . , 19-3 selected by a select pulse supplied from a horizontal shift register 10, respectively.
FIG. 2 is an example of the sectional structure of a prior art photodiode.
In FIG. 2, a reference numeral 20 denotes a P-type semiconductor substrate having uniform impurity concentration (about 1xc3x971015 cmxe2x88x923), a reference numeral 21 denotes a P-well formed by injecting ions of P-type impurities such as boron (B) (with a concentration of about 1xc3x971017 cmxe2x88x923), a reference numeral 22 denotes an N-type region formed by injecting ions of N-type impurities such as phosphorous (P) and reference numeral 23 denotes a depletion region at a PN junction.
In the structure shown in FIG. 2, the concentration of the P-type semiconductor substrate 20 is low (i.e., high resistance) and the concentration of the P-well 21 is higher than that of the P-type semiconductor substrate 20. With such a structure, diffusion current from the P-type semiconductor substrate 20 is high and a large amount of current is generated in the depletion region 23 since crystal defects are introduced into the P-well 21 as a result of ion implantation. This causes a problem that diode dark current which is the sum of the diffusion current and the generation current is high. The prior art structure also has a problem that, due to the above reason, photo-sensitivity is low and that the phenomenon of the leakage of signal charges into adjacent photodiodes (color crosstalk) increases.
The amplifying MOS image sensor shown in FIG. 1 which has amplifying transistors (2-1-1, . . . , 2-3-3) within a unit pixel, has characteristically high sensitivity. On the other hand, it has a disadvantage in that non-uniform gate threshold voltages appear as fixed pattern noise. To get rid of that disadvantage, there is known a method of providing noise cancelers 18-1, . . . , 18-3 on ends of vertical signal lines, respectively.
FIG. 3 shows a circuit diagram wherein the noise cancelers 18-1, . . . , 18-3 are provided. FIG. 4 shows a specific example of the noise canceler. The noise canceler 18 mainly comprises, for example, a clamp capacitance C1, a clamp transistor Tr1 for resetting a node connected to the clamp capacitance C1, a sample hold capacitance C2, a sample hold transistor Tr2 for reading a signal from the node connected to the clamp capacitance C1. The differential signal between a dark period and a bright period (light incidence period) is outputted to the vertical signal line 11 by the nose canceler. The noise cancel operation of the noise canceler is conducted for a horizontal blanking period or a short period of time such as 10.9 microseconds in a NTSC system and 3.77 microseconds in a High-Vision system. This requires therefore the transistors and capacitance which constitute the noise canceler 18 to have high-speed operation performance.
If a transistor is formed within the P-well shown in FIG. 2, the response speed of the transistor is determined by the product of the resistance of the P-well and the capacitance of the depletion layer of the source-drain region of the transistor due to the high resistance of the substrate. This is because current is supplied from a well contact formed on the P-well to the source-drain region in response to a variation in the potential of the transistor. In this case, the P-well may well have quite high concentration (or low resistance) to fasten the response speed. If so, however, it is difficult to control threshold voltage by ion implantation into the channel region of the transistor. The clamp capacitance C1 and the sample hold capacitance C2 shown in FIG. 4 are formed on an insulating film formed on the semiconductor substrate. The capacitance of the insulating film is, therefore, added to the capacitance C1 and C2 in series or in parallel. The response speed of capacitance of the insulating film is determined by the product of the P-well resistance and capacitance. For the same reason of the above-stated transistor, it is difficult to fasten the response speed.
Therefore, it is difficult to realize the high-speed operation of the transistor and the capacity which composes a noise canceler as far as it used the conventional wafer section structure shown in FIG. 2.
As described above, the prior art MOS-type solid-state image sensor has disadvantages in that dark current at the photoelectric conversion portion is high and that component noise during a dark period is large. It also has disadvantages of low sensitivity, color crosstalk and/or high degree of blooming. xe2x80x9cBloomingxe2x80x9d here is a phenomenon that signal charges are poured into adjacent pixels if intensifier light is incident. Moreover, the prior art MOS-type solid-state image sensor has a disadvantage in that it is difficult to realize the high-speed operation of the transistors which are the constituents of the noise canceler.
It is an object of the present invention to realize lower dark current (less dark time noise) at the photoelectric conversion portion, higher sensitivity, less color crosstalk and less blooming, and to provide a solid-state image sensor capable of realizing the high-speed operation of noise cancelers.
The present invention has taken the following measures to attain the above object.
A solid-state image sensor according to the present invention is characterized by comprising: a semiconductor substrate; a photoelectric conversion portion formed above the semiconductor substrate; and noise cancelers each formed, adjacent to the photoelectric conversion portion, on the semiconductor substrate through an insulating film, for removing noise of a signal read from the photoelectric conversion portion, wherein the semiconductor substrate has a conductive type opposite to a conductive type of a charge of the signal, and has a first region where concentration of impurities for determining the conductive type is high and a second region where concentration of the impurities on the first region is low.
Other solid-state image sensors according to the present invention is characterized by comprising a semiconductor substrate; a photoelectric conversion portion formed above the semiconductor substrate; a third region formed above the photoelectric conversion portion, having a same conductive type as that of a first region and having almost a same impurity concentration as that of the first region; and noise cancelers each formed, adjacent to the photoelectric conversion portion, on the semiconductor substrate through an insulating film, for removing noise of a signal read from the photoelectric conversion portion, wherein the semiconductor substrate has a conductive type opposite to a conductive type of an electric charge of the signal, and has the first region where concentration of impurities for determining the conductive type is high and a second region where concentration of the impurities on the first region is low.
The preferred modes of the above-stated solid-state image sensor are as follows:
(1) The impurity concentration in the first region is 1xc3x971018 cmxe2x88x923 or higher.
(2) The impurity concentration in the second region is gradually lower toward a surface of the semiconductor substrate.
(3) The second region is formed by epitaxial growth.
(4) The second region is formed by thermal solid-phase diffusion; and the impurities are diffused from the first region toward the second region to thereby provide the impurity concentration in the second region with a desired concentration gradient.
(5) A temperature for use in the solid-phase diffusion is between 1000xc2x0 and 1300xc2x0 C. and diffusion time is between 10 and 480 minutes.
In the solid-state image sensor according to the present invention, most parts of the semiconductor substrate act as a high concentrated impurity region and carriers having the same conductive type as that of signal charges in that region have shorter lifetime and less mobility. This results in the reduction of substrate diffusion current components, whereby dark current can be reduced.
In addition, by gradually making impurity concentration lower toward the surface of the semiconductor substrate in the low concentrated impurity region formed on the high concentration impurity region, a potential gradient is formed toward the surface of the semiconductor substrate. Therefore, the efficiency of collecting light converted signal charges is improved, thereby capable of realizing high sensitivity. By the similar reason, such a force as to direct signal charges toward the surface of the substrate in parallel along the potential gradient is exerted to thereby make it difficult to diffuse signal charges in lateral direction.
Following the above respects, it is possible to reduce the diffusion-led leakage of signal charges in the direction of adjacent pixels, that is, to reduce color crosstalk. Likewise, blooming can be reduced by the solid-state image sensor according to the present invention. This is because blooming is mainly caused by the diffusion of signal charges in lateral direction as in the case of the leakage or color crosstalk.
Moreover, if the low concentrated impurity region having the above-stated potential gradient is formed by epitaxial growth or thermal solid-phase growth and impurity concentration in the vicinity of the surface of the semiconductor substrate is set equal to P-well concentration, then it is possible to form an impurity region having the same concentration as that of the conventional P-well without ion implantation. Due to this, it is possible to reduce generation current components in the depletion region. Thus, the present invention can provide a solid-state image sensor capable of realizing low dark current, high sensitivity, less color crosstalk and less blooming.
In addition, even if the surface of the substrate is kept at low concentration to such an extent that the threshold voltage of the transistor can be controlled, it is possible to reduce resistance for determining the response speed of the transistors and capacitance which constitute noise cancelers by providing the substrate itself with high concentration. In other words, the present invention can provide a noise canceler capable of realizing high-speed operation.
The advantages of the present invention are as follows:
The present invention can provide a solid-state device capable of obtaining low dark current, high sensitivity and less color crosstalk (as well as less blooming). Furthermore, the present invention can realize a highly refined multiple-pixel device wherein fixed pattern noise is prevented.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.